Our prior work has shown that injection of oxalic acid can substantially increase mobilization of arsenic from sediments at the Vineland Superfund site (from laboratory to pilot field scales), potentially showing a way to greatly enhance the pump and treatment (P&T) methodology currently in use there as well as at other sites. Here, our overall aim is to partner with site managers who are very interested in evaluating how to scale-up and determine what portions of the site are most amenable to our oxalic acid accelerated P&T approach. Our primary hypothesis is that we can overcome spatial heterogeneities in both hydraulic flow-paths and arsenic geochemistry to allow the oxalic acid approach to dramatically increase the efficiency of P&T. A series of aims focused on field, laboratory and modeling experiments to be carried out in partnership with USEPA&USACE scientists will be used to determine an optimized combination of injection well density, timing, and reactant composition to allow us to plan and carry out a medium scale pilot study, where a key aim will be the use of continuous, high resolution geophysical resistivity survey methods to determine in realtime the advection and dispersion of injected oxalic acid. A recent Remedial Site Evaluation Review of the Vineland Superfund Site recommended that different approaches may be needed in different portions of the site and that remediation by in situ immobilization might provide cheaper and more effective strategies for some portions of the site. As such, we will also investigate immobilization approaches that focus on developing the in situ use of the mineral magnetite to immobilize arsenic; we hypothesize magnetite will be a good target mineral since it is stable under both oxic and reducing conditions found at many sites and has recently been shown to incorporate arsenic into its structure. Our aims here will be to carry out preliminary lab studies in order to predict when magnetite will form and when it will effectively result in a stable net sink of aqueous arsenic. Then a small pilot field experiment testing As immobilization with magnetite will be carried out in years 4-5. Finally we will use sediments collected from other Superfund sites (e.g. a landfill in Dover NH) to carry out preliminary laboratory studies on the transferability of the 2 methods.